As it is well-known, there have been many proposals concerning a method for producing methacrylic acid through the gas-phase catalytic oxidation of methacrolein by using a catalyst so far.
Heat accumulation takes place in the catalyst layer because the gas-phase catalytic oxidation is an exothermic reaction. A locally high temperature zone resulting from the heat accumulation is called a hot spot, and when the temperature of this zone becomes higher than it needs, an excessive oxidation reaction takes place so than the yield of the target product is lowered. Consequently, in the industrial enforcement of the aforementioned oxidation reaction, the temperature control of the hot spot is an important subject, and especially when the concentration of methacrolein in the raw gas is raised to increase the productivity, the temperature of the hot spot tends to become high so that a large restriction is forced upon the reaction conditions in the present situation.
Accordingly, there have been several proposals concerning a method for suppressing the temperature of the hot spot so far. For example, a method of packing a plurality of catalysts having different activity in a plurality of reaction zones to order of raise the activity from the inlet part to the outlet part for the raw gas (Patent document 1), a method of packing a catalyst which has a higher compositional ratio of phosphorous and a lower compositional ratio of arsenic nearer to the outlet side for the raw gas (Patent document 2), a method of packing a catalyst which contains a lesser amount of the characteristic element such as potassium nearer to the outlet side for the raw gas (Patent document 3), a method of packing the catalyst wherein a catalyst layer is divided into a plurality of reaction zones, and the activity of the first reaction zone of the inlet side for the reaction gas is adjusted to be higher than that of the second: reaction zone, and the activity of the third reaction zone and the zones thereafter are adjusted to be higher in ascending order (Patent document 4), and the like are exemplified.    Patent Document 1: Japanese Patent Application, First Publication No. Hei 4-210937    Patent Document 2: Japanese Patent Application, First Publication No. 2000-70721    Patent Document 3: Japanese Patent Application, First Publication No. 2003-171339    Patent Document 4: Japanese Patent Application, First Publication No. 2003-261501
These are the methods of suppressing the heat value of the reaction per unit volume by lowering the rate of reaction per unit volume at the inlet side for the raw gas in the catalyst layer of the reactor to result in lowering the temperature of the hot spot. Therefore, these are effective in improving the yield by suppressing the consecutive oxidation reaction and in lengthening the catalyst life by reducing the thermal load.
However, these methods only paid attention to suppressing the temperature of the hot spot so that the quantity of oxidation per unit mass of the catalyst in each reaction zone of the catalyst layer has not been controlled at all and consequently the distribution of the load of the oxidation reaction in the catalyst layer has not become uniform and it has been feared that a portion where the load of the oxidation reaction is high may occur. At this portion, the probability of the occurrence of failure in the reoxidation of the catalyst became high, so that deterioration of the catalyst was accelerated and it has been a concern that the life of the catalyst layer as a whole may be drastically shortened.
The present invention has been achieved by taking the above-mentioned problems into consideration and has objects to provide a method for producing methacrylic acid through the gas-phase catalytic oxidation of methacrolein with molecular oxygen in the presence of a solid oxidation catalyst by the use of a fixed-bed tubular reactor, wherein the local deterioration of the catalyst is suppressed and the catalyst is used stably for a long time by not only suppressing the temperature of the hot spot but also making the load of the oxidation reaction in the catalyst layer uniform, and to provide catalyst layers and a fixed-bed multitubular reactor.